agent_ui: Add floating user prompt navigation buttons and enable scrolling back and forth to user messages

[Tahinli]

Replace the single "Scroll To Most Recent User Prompt" button with previous/next navigation buttons that step through user messages. Move the thread controls bar from inline (scrolled with content) to a floating overlay anchored at the bottom-right of the conversation. Add a scroll-to-bottom button alongside the existing scroll-to-top.

Self-Review Checklist:

• [ X ] I've reviewed my own diff for quality, security, and reliability
• [ X ] Unsafe blocks (if any) have justifying comments
• [ X ] The content is consistent with the UI/UX checklist
• [ - ] Tests cover the new/changed behavior
• [ X ] Performance impact has been considered and is acceptable

Closes #52425

Release Notes:

• Floating panel is added.
• Previous,next and scroll to bottom buttons are added.
• Scroll to latest message button is removed but function is preserved since it's used in tests.